MEMS devices integrating thousands of complex and very sensitive free-to-move components integrating digital and/or analog CMOS control logic and/or high voltage CMOS drivers are used in many fields, such as performing sensing and/or actuation functions for consumer electronics, automotive and other high volume and low cost MEMS applications.
The integration of free-to-move mechanical devices that are in direct with ambient atmosphere creates a very serious challenge because these devices are typically very fragile and require a protective cover that protects against atmospherics debris. This protective cover needs to be integrated after these mechanical devices have been made free-to-move, thus preventing the use of the popular sacrificial layer approach that would become in direct physical contact with these free-to-move mechanical devices. Moreover, the protective cover needs to be integrated using low temperature processing because in most cases, these free-to-move mechanical devices cannot experience a high temperature exposure because of the various thermal coefficients of expansion of their constituting materials. A maximum exposure temperature of about 250° C. is typical for such sensitive MEMS devices.
Various methods are known in the prior art for making the protective layer. A first example involves the use of a sacrificial layer. A first example of a process to fabricate such a protection capsule is shown in U.S. Pat. No. 6,635,503. This process requires a “sacrificial layer” to be deposited over the free-to-move mechanical devices and under the protection capsule. Unfortunately this sacrificial material approach cannot be used because this sacrificial material would need to become in physical contact with the free-to-move mechanical devices, thus causing mechanical issues and possibly destruction of the free-to-move mechanical devices. Additional examples of such an approach are the following Prior Art documents: U.S. Pat. No 5,322,594 titled: “Manufacture of a One Piece Full Width Ink Jet Printing Bar and U.S. Pat. No. 6,902,656 titled: “Fabrication of Microstructures with Vacuum Sealed Cavities
A second approach involves the use of a frit glass. An example of such a process to fabricate such a protection capsule is shown in FIG. 1. This process uses a precision screen to deposit a slurry comprising organic materials and a frit glass containing filler onto the protection capsule. Following screen printing and heating to a high enough temperature to volatilize the organic materials, the protection cap is contacted and pressed against the MEMS wafer while heating to a high temperature preferably about 350° C.-475° C. as to exceed the softening point of the frit glass material and allow the thermo-compression bonding of the protection cap onto the MEMS wafer in such a way that no frit glass touches the free-to-move mechanical devices upon contact. Unfortunately, such a bond temperature of about 350° C.-475° C. exceeds the maximum temperature requirement of about 250° C. of such sensitive MEMS devices. Examples of such an approach are the following Prior Art documents:                U.S. Pat. No. 5,323,051 titled ‘Semiconductor wafer level package’;        U.S. Pat. No. 6,465,281 titled ‘Method of manufacturing a semiconductor wafer level package’;        Gary Li, Ampere A. Tseng, ‘Low stress packaging of a micromachined accelerometer’, IEEE Transactions on electronics packaging manufacturing, Vol. 24, No. 1, January 2001;        
A third approach for fabricating such a protection capsule involves anodic bonding of the protection capsule. This process requires the sodium atoms of the protection capsule made of sodium-based silica glass (such as Corning Glass‘Pyrex™ 7740) to be diffused at a temperature of about 350-450° C. and under a high electrical field created by a negative voltage of about 1000-2000V applied between the Pyrex™ protection capsule and the MEMS wafer incorporating the free-to-move mechanical devices as to allow sodium displacement of the silicon atoms of the Pyrex™ protection capsule and the anodic bonding to the MEMS wafer. Unfortunately, the anodic bond temperature of about 350° C.-475° C. again exceeds the maximum temperature requirement of about 250° C. of such sensitive MEMS devices. More, the sodium being an undesirable mobile ion inducing threshold voltage shifts of CMOS and high-voltage CMOS devices the use of anodic bonding is to be avoided for the production of complex MEMS micro-devices are formed by integrating very sensitive free-to-move mechanical devices, digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing sensing and/or actuation functions. An example of such an approach is disclosed in U.S. Pat. No. 5,952,572 titled ‘Angular rate sensor and acceleration sensor’ (Matsushita Electric Industrial Co., Ltd.)
A fourth example of a process to fabricate such a protection capsule involving eutectic bonding is disclosed in U.S. Pat. No. 5,668,033. This process requires the bonding of a previously machined cover wafer onto the MEMS wafer using gold-silicon or gold-polysilicon eutectic bonding at a temperature of more then about 360° C. Again, such a bond temperature exceeds the maximum temperature requirement of about 250° C. of such sensitive MEMS devices. More, such an eutectic bonding approach imposes the use of gold, a material proscribed in CMOS manufacturing facilities where complex MEMS micro-devices are formed by integrating very sensitive free-to-move mechanical devices, digital and/or analog CMOS control logic and/or high voltage CMOS drivers capable of performing sensing and/or actuation functions.
A fifth example of a process to fabricate such a protection capsule involving soldering, brazing or direct metal bonding is shown in U.S. Pat. No. 6,297,072. This process requires the soldering of a first wafer to a second wafer integrating a MEMS device by soldering, brazing or direct metal bonding the protection capsule to the MEMS wafers integrating the free-to-move mechanical devices. This technique requires suitable solderable under-bump metals (UBM), brazing metal patterns or planarized metal patterns to be present on the protection capsule and on the MEMS wafer integrating the free-to-move mechanical devices as to allow the soldering, brazing or direct metal bonding. Although possible, this technique involves more steps and is a more complex and expensive process that the one presented in the following patent application. Additional examples of such an approach are shown in:                U.S. Patent Application 20040067604 titled: ‘Wafer level packaging technique for microdevices’        U.S. Patent Application 20050142685 titled: ‘Hermetic wafer-level packaging for MEMS devices with low-temperature metallurgy’        U.S. Patent Application 20050161795 titled: ‘Room Temperature Direct Wafer Bonding’        
A sixth example of a process to fabricate such a protection capsule, involving photopolymer bonding, is shown in U.S. Pat. No. 5,907,333. This process requires a photosensitive polymer to be spun-on onto the wafer integrating the free-to-move mechanical devices before being exposed and developed as to form a basis onto which a machined protection capsule is bonded. Unfortunately this approach cannot be used because this spin-on photopolymer would need to become in physical contact with the free-to-move mechanical devices, thus causing mechanical issues and possibly desctuction of the free-to-move mechanical devices. Additional examples of such an approach are shown in:                U.S. Pat. No. 6,193,359 titled: ‘Ink Jet Print Head Containing a Radiation Curable Resin Layer’        F. Niklaus, P. Enoksson, P. Griss, E. Kälvesten and G. Stemme, ‘Low-Temperature Wafer-Level Transfer Bonding’, Journal of Microelectromechanical Systems, Vol. 10, No. 4, December 2001, pp. 525-531        J. Oberhammer and G. Stemme, ‘Contact printing for improved bond-strength of patterned adhesive full-wafer bonded 0-level packages’, 17th IEEE International Conference on Micro Electro Mechanical Systems, Maastricht, The Netherlands, Jan. 25-29, 2004, pp. 713-716        
A seventh example of a process to fabricate such a protection capsule involving a monolithic photopolymer is shown in U.S. Pat. No. 5,458,254. This process agains require a photosensitive polymer to be spin-on directly onto the wafer integrating the free-to-move mechanical devices before being exposed and developed. Unfortunately this direct spin-on approach cannot be used because this spin-on photopolymer would need to become in physical contact with the free-to-move mechanical devices, thus causing mechanical issues and possibly destruction of the free-to-move mechanical devices. Additional examples of such an approach are shown in:                U.S. Pat. No. 6,162,589 titled: ‘Direct Imaging Polymer Fluid Jet Orifice’        U.S. Pat. No. 6,303,274 titled: ‘Ink Chamber and Orifice Shaoe Variations in an Ink-Jet Orifice Plate’        U.S. Pat. No. 6,305,790 titled: ‘Fully Integratable Thermal InkJet Printhead Having Multiple Ink Feed Holes per Nozzle’        U.S. Pat. No. 6,336,714 titled: ‘Fully Integratable Thermal InkJet Printhead Having Thin Film Layer Shelf’        U.S. Pat. No. 6,419,346 titled: ‘Two-Step Trench Etch for a Fully Integrated Thermal InkJet Printhead’        U.S. Pat. No. 6,447,102 titled: ‘Direct Imaging Polymer Fluid Jet Orifice’        U.S. Pat. No. 6,450,622 titled: ‘Fluid Ejection Device’        U.S. Pat. No. 6,454,393 titled: ‘Chamber and Orifice Shape Variations in an Orifice Plate’        U.S. Pat. No. 6,481,832 titled: ‘Fluid-Jet Ejection Device’        U.S. Pat. No. 6,517,735 titled: ‘Ink Feed Trench Etch Technique for a Fully Integrated Thermal Inkjet Printhead’        U.S. Pat. No. 6,520,627 titled: ‘Direct Imaging Polymer Fluid Jet Orifice’        U.S. Pat. No. 6,520,628 titled: ‘Fluid Ejection Device With Substrate Having a Fluid Firing Device and a Fluid Reservoir on a First Substrate Thereof’        U.S. Pat. No. 6,527,368 titled: ‘Layer With Discontinuity Over Fluid Slot’        U.S. Pat. No. 6,543,884 titled: ‘Fully Integratable Thermal InkJet Printhead Having Etched Back PSG Layer’        U.S. Pat. No. 6,626,523 titled: ‘Printhead Having a Thin Film Membrane With a Floating Section’        
An eighth example of a process to fabricate such a protection capsule involving the transfer technique using wax is shown in FIG. 2. This process requires the protection capsule to be first bonded to a CARRIER wafer using a low temperature wax. Then, a photosensitive benzocyclobutene, BCB, is spun-on, exposed and developed as to define a bond pattern. Then the BCB of the protection capsule is properly aligned and bonded to the MEMS wafer integrating the free-to-move mechanical devices. Then the wax of the CARRIER wafer is heated above its melting point as to detach the BCB bonded protection capsule to the MEMS wafer integrating the free-to-move mechanical devices. This protection capsule transfer technique is very interesting because it prevents a direct physical contact of the BCB with the free-to-move mechanical devices, and because the polymer bonding (250° C.) and the wax de-bonding (150° C.) are both performed at temperatures lower then the maximum temperature requirement of about 250° C. for such sensitive MEMS devices. This technique requires a lot of processing (Wax coating/drying on a CARRIER wafer+BCB coating/exposure/develop over wax+BCB align/bond on MEMS wafer+Wax de-bonding bonding. More, it limits the protection capsule to a monolith that prevents the definition of individual protection capsules on the individual free-to-move mechanical devices of the MEMS wafers. Typical MEMS wafers may contain a few thousand MEMS devices when these target consumer electronics, automotive and other high volume and low cost applications. In that case, the transferred monolithic protection capsule defining cavities above the individual free-to-move mechanical devices of the MEMS wafers yet does not provide the thousands of individual protection capsules above the thousands of individual free-to-move mechanical devices of the MEMS wafers. More processing of the monolith (such as wet etching, dry etching or precision sawing) is then required to achieve this goal. The present patent application provides a much simpler and much less expensive technique to achieve this goal of providing individual protection capsules above the individual free-to-move mechanical devices of the MEMS wafers. An example of such an approach in shown in the following Prior Art document:                A. Jourdain, X. Rottenberg, G. Carchon and H.A.C. Tilmanstitled, ‘Optimization of 0-Level Packaging for RF-MEMS Devices’, Transducers 2003, The 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003, pp. 1915-1918        
A ninth example of a process to fabricate such a protection capsule using a transferred parylene membrane is shown in FIG. 3, where step a illustrates the carrier wafer; step b the deposit a sacrificial layer; step c the deposit of parylene on sacrificial layer; step d the patterning of parylene to expose sacrificial layer; step e the removal of the sacrificial layer to suspend parylene; step f the deposition of parylene on a MEMS wafer; step g the alignment and bonding of a suspended parylene layer to MEMS wafer's parylene layer; and step h the detachment of the MEMS wafer from the carrier.
The parylene layer is transferred to MEMS wafer. This particular process uses a carrier wafer coated with 1.3 um of AZ1813 sacrificial photoresist over which a 0.38 um thick layer of parylene is deposited and patterned as to expose the underlying layer of parylene. Following local etch of the exposed parylene the underlying sacrificial photoresist is dissolved in acetone as to leave a free-standing pattern of parylene on the carrier wafer. The patterned MEMS wafer integrating the free-to-move mechanical devices is coated with another layer of 0.38 um thick layer of parylene and is aligned and pressed against the free standing pattern of parylene on the carrier wafer while heating at 230° C. under a vaccum of 1.5*10−4 Torr. The two parylene layers will polymerize together and will result in a bond strength of 3.6 MPa. This transferred parylene process is undesirable because the poor stiffness of the 0.38 um thick free-standing pattern of parylene on the CARRIER wafer only allows small sizes membranes to be transferred. More, one of the 0.38 um thick layer of parylene comes in physical contact with the free-to-move mechanical devices, thus causing mechanical issues and possibly desctuction of the free-to-move mechanical devices. Finally the achieved bond strength of 3.6 MPa is not high enough for many MEMS applications. An example of such an approach in shown in the following Prior Art document:                H. S. Kim and K. Najafi, ‘Wafer Bonding Using Parylene and Wafer-Level Transfer of Free-Standing Parylene Membranes’, Transducers 2003, The 12th International Conference on Solid State Sensors, Actuators and Microsystems, Boston, Jun. 8-12, 2003, pp. 790-793        